Radiolabeled tumor cell extracts will be used in quantitative immunoprecipitation assays to estimate the amount of tumor cell-associated antigens recognized by antibodies in unblocked sera from normal donors and from patients with malignant melanoma and other malignancies. The immunoprecipitates will be analyzed by polyacrylamide gel electrophoresis (PAGE) followed by radioautography to determine the number and molecular size of the antigens recognized by these sera. The proteins identified by these methods will be compared with those identified by isoelectric focusing followed by PAGE of the original tumor cell extracts. In this way, we will be able to identify molecular characteristics of distinctive tumor cell surface antigens which are recognized by human alloantisera as a preliminary to their isolation for use in diagnostic radioimmunoassays.